Apparatus for producing a hollow electron beam



. Aug. 20, 1957 v H. F. WEBSTER APPARATUS FOR PRODUCING A HOLLOW ELECTRON BEAM Filed Jan. l2,` 1955 ,////////////////////7//f////l//l W:1112::z:2222222322? :,l 'Y

United States Patent O APPARATUS FOR PRODUCING A HOLLOW ELECTRON BEAM Harold F. Webster, Schenectady, N. Y., assignor to Gen` eral Electric Company, a corporation of New York Application January 12, 1955, Serial No. 481,375 6 Claims. (Cl. 313-157) My invention relates to apparatus for producing a hollow electron beam and particularly to apparatus for producing such a beam which is well Asuited for use in high frequency devices.

In electron discharge equipment of the beam type, the cathode and associated electrodes for producing the beam are very important to both the life and the quality of performance of -the equipment. In high frequency equipment in which an electron beam is coupled to a conductive structure, it is desirable that the beam be dense, uniform in density and velocity over the cross section of the beam and concentrated essentially at a surface (the surface at which the coupling is to take place). The density of the beam contributes to the strength of the coupling with the high frequency structure while the uniformity and concentration of the beam at a vsurface contribute to the ability of the beam to produce only the desired excitation of the high frequency structure and thus to the quality of the performance of the equipment.

In accordance with my invention, I provide apparatus including an electron gun which produces a hollow beam having these characteristics and which is further characterized by long life of the emitter (cathode per se). Specifcally, I have found that a hollow cathode of proper configuration having an opening therein, when operated in a highly evacuated envelope, tends to produce a hollow beam of electrons, that is, a beam in which substantially all of the electrons are concentrated in an annulus corre sponding in shape and size to the opening. By immersing the cathode in a magnetic eld parallel in direction to the beam produced, the relatively low electron density in the center of the beam is reduced to a negligible value leaving a rather clearly defined annular beam having a total current substantially equal to that produced without the magnetic field. My invention also includes the provision of means for minimizing the emission of electrons from the exterior of the hollow cathode which would tend to prevent the production of a well-delined hollow electron beam.

It is'an object of my invention to provide a new and improved electron discharge apparatus for producing a hollow electron beam.

It is a further object of my invention to provide an electron gun characterized by longer operating life and improved electron beam producing characteristics.

Further objects and advantages of my invention will become apparent as the following description proceeds, reference being had to the accompanying drawing and its scope will be pointed out in the appended claims. In the drawing, Fig. 1 is a longitudinal view in section of electron discharge apparatus embodying my invention, and Fig. 2 illustrates schematically a transverse section of the electron beam produced by the apparatus of Fig. l.

Referring now to the drawing, I have shown my invention embodied in electric discharge apparatus of the beam type and including a highly evacuated cylindrical envelope 1 in which the electrodes are mounted in spaced y,

2,803,772 Patented Aug. 20, 1957 relation along the axis thereof. An electron source s provided by a cathode member 2 which is in the form of a hollow cylinder closed at its lower end by plate 3. In its preferred form, the upper end is an outwardly extending conical portion 4 which is truncated to provide a relatively small centrally located circular opening 5. The interior wall of the cathode body or at least the portion of the wall surrounding the opening 5 is coated with a suitable electron emission enhancing coating of a type well known in the art.

In the preparation of the cathode, the emitting materials, such as one or more of the alkaline earth oxides, are mixed with a suitable binder or carrier to form a slurry which is introduced into the cathode through the opening 5. The slurry is spread around and the excess poured off to leave a layer 6 of the emitting material on the interior surface of the cathode cylinder and particularly on the inner surface of the conical portion.

In order to raise the temperature of the cathode 2 sufciently to produce thermionic emission from the coating 6, I provide a suitable resistance-type heater enclosing the cathode cylinder 2 and illustrated schematically by the heater wires 7. These wires are enclosed in a suitable heat conserving shield 8 which is generally cup-shaped to conform with the shape of the heater. As illustrated, the heater is insulated from both the shield 8 and the cathode 2 by refractory insulating material 9. The cathode assembly thus far described is supported from a plurality of lead-in conductors 10 sealed through a press 11 in the lower end of the envelope l by conductors 12-14, inclusive. Conductor 12 supports the cathode 2 and conductors 13 and 14 are joined to opposite ends of the heater wire 7 As is apparent from the drawing, bonded to the heat shield 8 to port and to connect heater.

An accelerating electrode or anode 15 is supported from the remaining lead-in conductor 10 by conductor 16. Specifically anode 15 is annular in shape and is supported in parallel relation to the plane of the cathode opening 5 andswith the opening 17 therein in alignment with opening As a means of minimizing the emission of electrons from the exterior of the cathode 2, I provide an electrostatic shield 1S between the cathode and anode which operates at cathode potential and at a substantially lower temperature than the cathode. The shield is in the form of an annular member 18 mounted between the anode and cathode and supported from the same conductor 12 which supports the cathode 2. The opening 19 in the shield is a little larger than the end of the cathode so that the conduction of heat to the shield is minimized. It is apparent that the shield 18 is electrostatically a part of the cathode structure extending outwardly in essentially the same plane as the opening 5 of the cathode. It is also apparent that the shield may be supported directly from the conical portion of the cathode by a material having relatively low thermal conductivity to provide the desired lower operating temperature of the shield 18.

At the opposite end of the envelope i, a collector electrode 20 is supported in alignment with the openings in the cathode 2, shield 18 and anode 15 by means of lead-in conductor 21. A suitable high frequency conductive structure 22 is supported between the anode 15 and collector 20 for interaction with the electron beam. As illustrated, this structure may be the helix of a traveling wave tube.

Means for `collimating the electrons of the beam are provided by an electromagnet including aV cylindrical support 23 of insulating material and a coil illustrated the conductor 14 is also provide mechanical sup- A1t electrically with one end of the schematically at 24. In accordance with the 'present ditions.

invention, the field produced by the electromagnet also cooperates with the cathode to produce a hollow beam of electrons. It is apparent that, for the coil 24 to be effective for this latter purpose-the, shield-8 andthe cathode'bodymust be non-,magnetic ,under joperating. convThis may be .accomplished by ut`ilizing nonmagneticmaterial, such as tantalumwor metal which becomes non-magnetic at a temperature lower than its normal operating temperature. 1 In accordancewith one form ofthe device on which tests were conducted, the

-shieldewas formed of-molybdenum and the cathode of nickel which'y becomes non-magneticat 36.0 C. Since -eld'funder operating conditions.

As--willbe readily understoodby those skilledin the art, thetube may be operated` bythe application of suitable-potential to the conductorslt) andjv21 to heat the cathode-and to acceleratethe electrons` toward the collector electrode/211. AFor example, the anode andcollector electrode 20 may be maintained atea voltage in the order of 100 to 1000 volts-D. C. with respect to the cathode H2 and the shield i8. The magnetic'tield may be-in-the orderoflOO to A300 gauss.

"Withthisv construction, electrons emitted from the interiorv-ofthe-cathode are accelerated toward Vthe collector 21 ina=beam having an exterior corresponding generally tothe shape and size ofthe opening in the cathode. Withoutthe--magneticdield and with the configuration of the cathode and shield-illustrated, the density of the electronslis muchlower in the'center thanit is toward the outersurfaeefof thebeam. The electrons from the center, however, tend-tolhave a-higher velocity than'those at the outeredge and these are accordingly undesirable. `VvWhen the cathode is combined-with the axial magnetic field, theelectrons in the central `portion of the beam are essentiallyeliminated and the `Vtotalcurrent ofthe beam has substantially the same total value as it does without the magnetic field. Accordingly, the combination provided by this invention results in a rather well-defined annular beam =of-high current ydensity and uniform velocity at a givencross section. The hollow cathode, of course, has the.advantagespreviously observed with respectto hollow emitting structures, lnamely, -that -of conserving the emission enhancing material, minimizing the contamination of ,the other parts with this material and also minimizing the. contamination off` the emitting material by -anymate rial. driven off-from the other electrodes'during operation of the fdevice.

While Ihaveshowniand -described aparticular embodiment of :my invention, .it willbe apparent to those skilled inthe art that modifications `may be made without departing lfrom my invention and l, therefore, .aim `bythe appendedclaimsto cover any `such modifications as fall within the true spirit and scope of. my invention.

lWhat I Lclaim as .new and desire .to secure by Letters PatentY ofthe lUnited States is:

.1. Apparatusfor producing a hollow beam of electrons comprisingan evacuated envelopeazplnrality.of. electrodes includingacathode, `an anode having an opening therethroughand a collectorrelectrode supported vin alignment within said envelope .in the order named, said cathode comprising a hollow conductive member having an aperturelinthe side y:thereof toward said anodewith a maximum dimension substantially less than the maximum transverse ydimension of said cathode to provide `an internal surface ,around said aperture facing away from said anode, an electron emission enhancing coating on A,said internal .-surface, means vfor heating said Acathode and means surrounding at least said anode and said cathode for producing a magnetic field parallel to the path between the aperture in said cathode and said collector, said ,cathode being non-magnetic under operating con-l ditions So that said magnetic eld extends within .said .Cathode 2. Apparatus for producing a hollow beam of electrons comprising an evacuated envelope, a plurality of electrodes including a cathode, an anode and a collector electrode supported within said envelope in spaced relation along the path of the electron beam to be produced and in the order named, said cathode comprising a hollow cylindrical member with a conical end facing said anode, said end being truncated to provide a small centrally locatedopening, an electron emission .enhancing `coating on the interior :surface of said end surrounding said opening, means for heating said cathode to a temperaturesutcient `tojproduce emission from 'said' coating and to render said cathode non-magnetic, and means surroundingsaidcathlode for producing a magnetic field within said cathode parallel to the axis thereof.

3. Apparatus for producing a hollow beam of electrons comprising an evacuated envelope, a plurality of electrodesfincluding a cathode,van anode and. acollector elec- .trodey supported with,-,said' .env,elope `in` ,spacedy 'relation alongthe path .ofthe rvelectron beam. to be `producedzin the order named, said cathode comprisingy a 4hollow cylindrical member withan end.,slopinginwardlytand toward saidanode and defining asmall .centrally located opening,

an` electron emission enhancing coating Von the interior .surface'of saidfend surrounding said opening, means for heating said cathode .to a temperature sufiicientto produce emission from said coating, said cathode being nommagnetic under operatingconditiong'and means surrounding .said electrodes for producing a magnetic'field within said cathode; :parallel to the axisthereot.

`4. An electrongunY forV producing .a hollowelectron beam comprising ahollowrcjylindrical cathodehaving an outwardly extendingV conical', end, said, conical endbeing truncated; to` provide.. a. .centrally located"openingstherein, an 'electronemission enhancingcoating on thesinterior surface of said end surrounding .saidcopening,. avplanar conductive member supportedadjacent said end in heat insulated'relationswith respect .to'said cathode and havingan .aperture-registering withtthei aperture .invsaid end, said member. being electrically. connectedzto; said `cathode and -an aperturedanode supportedin parallel .and spaced' relation with. respectto vsaid .planar member.

5. -A hollowcathode comprisinga hollowcylindrical member withanaend 4sloping inwardly andatowardsaid anode fand. detining .a .small centrally? located: opening, .an electron emission enhancing coating on the interior surface of saidend-.zsurroundingsaid opening,.means forheating saidwcathodeto .a .temperature sutiicientto produce emission from^said coating, and aplanarlshieldy memberisupported in perpendicularrelationl to theaxisof saidcylindricahmember in substantially theplane of saidopening andforming electricallya` part of said. cathode.

6. A hollow .cathode comprisingv a hollow :cylindrical member with an` end isloping inwardly .and toward said anodeanddefining a. small centrally located opening,an electron emission enhancing coating on the interior-surface or said endnsurrounding said opening, meansforheating said cathode'to artemperature'sufiicientto-v-produce emission from said coating, an apertured .planar shield member, and means supporting said shield member' from said cylindrical member inperpendicular. relationto-the axis thereof insubstantially the plane of said openingiand said last-mentioned means Aconductinga minimum ofheat from Asaid cylindrical memherzto saidzshield: member.

References.Cited inthe file of this .patent UNITED *STATESy PATENTS 2,034,572 Found Mar. n.17, 1936 2,038,341 Bruche .Apr.,2l, 1.936 2,099,846 ,Farnsworth ..Nov.w23, 1937 2,124,401 ,Heimann July .19,1938 `2.632,13() Y .Hull .Man 17, 1953 

